In the international patent publication WO 03/095198, which is hereby incorporated by reference, a packaging material laminate is described which comprises at least one layer comprising magnetizable particles. The laminate is of the type used for manufacturing of for example liquid food packages, and generally comprises a layer of paper or carton, layers of plastic and barriers, such as for example oxygen barriers. One of the outer layers is normally a sealable layer of a thermoplastic material which is used when sealing one laminate to another. Using thermoplastic layers is known in the art and will not be further described herein.
The magnetizable particles can for example be magnetite, Fe3O4 and have a mean size of about 0.5 μm. Other materials and particle sizes can of course also be used. There exist other materials, such as for example maghemite Fe2O3, as well as other particles sizes (larger and smaller). Some may give higher seal heating power. However, care should be taken when choosing particles. Some particles can not to be used in food packaging due to legislation; others involve higher costs due to their manufacturing. At present, smaller particles than 0.5 μm need an expensive chemical manufacturing process, whereas larger particles can be manufactured mechanically by sifting.
The magnetizable particles are dispersed in any of the layers of the packaging material laminate, preferably in one of the plastic layers. Alternatively, they can be applied in a printing ink or a hot melt, which in turn is applied to the packaging material in for example a sealing zone, as described in the Swedish Application Nos. 0501409-7 and No. 0501408-9.
A packaging material laminate that comprises magnetizable particles can be sealed to another packaging material laminate using heat generated by magnetic hysteresis losses. By applying an alternating magnetic field near a sealing zone of the first and second laminates the magnetic material will be magnetized according to the hysteresis loop in FIG. 1a. The vertical axis represents the magnetic moment B in the material and the horizontal axis represents the applied magnetic field H. The area enclosed by the loop represents the energy which is generated in the material due to the magnetizable particles. Since this is the energy that will be used to melt the outer sealable layers of the laminates to thereby create the seal, it is understood maximizing the hysteresis loop area will optimize the sealing process.
Generally, energy and sealing time are two parameters for controlling a sealing process. If the sealing energy is reduced, the sealing time will have to be increased, and vice versa. The same applies for magnetic hysteresis sealing. The larger the hysteresis loop area can be made, the shorter sealing time is needed (provided that the amount of particles in the laminates is the same). In a high speed packaging machine the sealing time is crucial. Then, if magnetic hysteresis sealing should be considered as a possible alternative to other sealing techniques, such as for instance induction sealing or ultrasonic sealing, the sealing time needed ought not to exceed the time required by the other techniques.
The hysteresis energy generation can be controlled in substantially two ways. One way is to increase the hysteresis loop area. It can be accomplished by increasing the applied magnetic field H until a level of magnetic saturation is reached in the particles of the laminates. The magnetic saturation level S is shown in FIG. 1b. However, increasing the magnetic field H above the saturation level S will not extend the hysteresis loop area.
Another way is to increase the frequency of the applied alternating magnetic field. Each cycle gives rise to energy generation corresponding to the hysteresis loop area, and by increasing the number of cycles per time unit the total amount of energy generated is increased. Hence, as an example, 1 Hz gives an energy contribution of one loop area per second, whereas 2 Hz will give a double energy contribution per second.
In this context it should be mentioned that authorities regulate the amount of electromagnetic radiation that can be emitted and which frequency bands are open for public use. In some frequency band intervals the use is restricted. In Europe this is presently controlled by the EMC Directive.
As mentioned above there are regulations on the amount of radiation that can be emitted. If exceeding the allowed values the device or machine needs to be shielded off from the surrounding environment. Such shielding is generally accomplished by mechanically encapsulating the device or machine in which the high frequency is used. However, it is known that emissions from high frequency devices are more difficult to shield off since they are more likely to slip out through any tiny opening in the shield. Thus, for practical and economic reasons it is preferred to use low frequencies. At present, an interval between 0.5-5 MHz is preferred. Hence, it is important to be able to optimize the hysteresis loop area not to be forced to increase the frequency.